Re: [RFC] Request API and V4L2 capabilities

Hans Verkuil
2018-08-06T11:31:54Z

On 08/06/2018 11:13 AM, Paul Kocialkowski wrote:
> Hi,
>
> On Mon, 2018-08-06 at 10:32 +0200, Hans Verkuil wrote:
>> On 08/06/2018 10:16 AM, Paul Kocialkowski wrote:
>>> On Sat, 2018-08-04 at 15:50 +0200, Hans Verkuil wrote:
>>>> Regarding point 3: I think this should be documented next to the pixel format. I.e.
>>>> the MPEG-2 Slice format used by the stateless cedrus codec requires the request API
>>>> and that two MPEG-2 controls (slice params and quantization matrices) must be present
>>>> in each request.
>>>>
>>>> I am not sure a control flag (e.g. V4L2_CTRL_FLAG_REQUIRED_IN_REQ) is needed here.
>>>> It's really implied by the fact that you use a stateless codec. It doesn't help
>>>> generic applications like v4l2-ctl or qv4l2 either since in order to support
>>>> stateless codecs they will have to know about the details of these controls anyway.
>>>>
>>>> So I am inclined to say that it is not necessary to expose this information in
>>>> the API, but it has to be documented together with the pixel format documentation.
>>>
>>> I think this is affected by considerations about codec profile/level
>>> support. More specifically, some controls will only be required for
>>> supporting advanced codec profiles/levels, so they can only be
>>> explicitly marked with appropriate flags by the driver when the target
>>> profile/level is known. And I don't think it would be sane for userspace
>>> to explicitly set what profile/level it's aiming at. As a result, I
>>> don't think we can explicitly mark controls as required or optional.
>>>
>>> I also like the idea that it should instead be implicit and that the
>>> documentation should detail which specific stateless metadata controls
>>> are required for a given profile/level.
>>>
>>> As for controls validation, the approach followed in the Cedrus driver
>>> is to check that the most basic controls are filled and allow having
>>> missing controls for those that match advanced profiles.
>>>
>>> Since this approach feels somewhat generic enough to be applied to all
>>> stateless VPU drivers, maybe this should be made a helper in the
>>> framework?
>>
>> Sounds reasonable. Not sure if it will be in the first version, but it is
>> easy to add later.
>
> Definitely, I don't think this is such a high priority for now either.
>
>>> In addition, I see a need for exposing the maximum profile/level that
>>> the driver supports for decoding. I would suggest reusing the already-
>>> existing dedicated controls used for encoding for this purpose. For
>>> decoders, they would be used to expose the (read-only) maximum
>>> profile/level that is supported by the hardware and keep using them as a
>>> settable value in a range (matching the level of support) for encoders.
>>>
>>> This is necessary for userspace to determine whether a given video can
>>> be decoded in hardware or not. Instead of half-way decoding the video
>>> (ending up in funky results), this would easily allow skipping hardware
>>> decoding and e.g. falling back on software decoding.
>>
>> I think it might be better to expose this through new read-only bitmask
>> controls: i.e. a bitmask containing the supported profiles and levels.
>
> It seems that this is more or less what the coda driver is doing for
> decoding actually, although it uses a menu control between min/max
> supported profile/levels, with a mask to "blacklist" the unsupported
> values. Then, the V4L2_CTRL_FLAG_READ_ONLY flag is set to keep the
> control read-only.
>
>> Reusing the existing controls for a decoder is odd since there is not
>> really a concept of a 'current' value since you just want to report what
>> is supported. And I am not sure if all decoders can report the profile
>> or level that they detect.
>
> Is that really a problem when the READ_ONLY flag is set? I thought it
> was designed to fit this specific case, when the driver reports a value
> that userspace cannot affect.

Well, for read-only menu controls the current value of the control would
have to indicate what the current profile/level is that is being decoded.

That's not really relevant since what you want is just to query the
supported profiles/levels. A read-only bitmask control is the fastest
method (if only because using a menu control requires the application to
enumerate all possibilities with QUERYMENU).

>
> Otherwise, I agree that having a bitmask type would be a better fit, but
> I think it would be beneficial to keep the already-defined control and
> associated values, which implies using the menu control type for both
> encoders and decoders.
>
> If this is not an option, I would be in favour of adding per-codec read-
> only bitmask controls (e.g. for H264 something like
> V4L2_CID_MPEG_VIDEO_H264_PROFILE_SUPPORT) that expose the already-
> existing profile/level definitions as bit identifiers (a bit like coda
> is using them to craft a mask for the menu items to blacklist) for
> decoding only.

That's what I have in mind, yes. I'd like Tomasz' input as well, though.

Regards,

Hans

> What do you think?
>
> Cheers,
>
> Paul
>